Assert Yourself
by Devi-Yosh
Summary: Sans was sick and tired of his brother Papyrus. He never gave him the credit he rightfully deserved whenever he did anything right, and whenever he was caught doing something wrong - which is what nearly always happens - he gets chewed out for being lazy and useless. Ever since then, he has been plotting on how to get back at his brother. (Underfell AU / tickle fic / mild language)


Sans was sick and tired of his brother Papyrus. He never gave him the credit he rightfully deserved whenever he did anything right, and whenever he was caught doing something wrong - which is what nearly always happens - he gets chewed out for being lazy and useless. He just can't seem to do anything right for his younger brother - his _younger_ brother! If anyone's supposed to be bossing anyone else around, it should be Sans!

Papyrus always claims that he's just trying to help the elder brother toughen up by treating him the way he does, but Sans just knows that's an just an excuse to keep doing it. Stupidly, he believed this for a long while and allowed Papyrus to keep treating him poorly, but until just recently when one of their typical heated arguments became physical he was starting to have second thoughts on what he truly believed came out of that little shit hole the taller skeletal monster calls a mouth.

Apparently Sans had done something Papyrus didn't approve of again by taking a small nap at his sentry post after working nonstop on trying to set up bear traps to catch a human - which, let's face it, there never _will_ be any damn humans to fall down here - and Sans, exhausted from all his hard work, tried to defend himself. Of course, Papyrus didn't believe him. Even after Sans showed him the traps, he decided to complain about the small number of them. They were freakin' difficult to set up and Undyne paid him only so much for his work, give him a break for crying out loud!

After the arguing turned into a verbal war, the shorter skeletal monster ended up pushing his younger brother, which he learned too late how big a mistake that was. Papyrus retaliated by summoning a red bone and slamming it down on Sans' cranium. Of course it took away a few fractions of HP, but not the entire 1 HP; he was left with roughly 0.2. After that, Sans stormed off, sporting a tiny crack on his skull which leaked a tiny bit of mustard. Papyrus demanded for him to stop storming off, but he was so angry he didn't even hear his brother call to him.

Ever since then, Sans had been plotting how to get back at his brother for all the pain and misery he caused him. He decided not to cause any kind of physical harm to him; he didn't want Papyrus to retaliate the same way, especially considering his HP is only at 0.8 at the moment. He had to come up with a method to really teach his younger brother a lesson without actually hurting him, something he could easily recover from.

That's when the idea struck him. It was very childish and mediocre, but it could be very humiliating on Papyrus' behalf. Plus, it'd be nice to win a fight for once, and Sans would have to make sure he couldn't retaliate.

Sans would tickle Papyrus into submission and become the next big-shot of the Underground. Just wait until the monsters hear that the Great and Terrible Papyrus, the most intimidating and cruel monster in the world - even worse than King Asgore himself - gave into his brother over tickling!

Revenge was going to be sweet...

* * *

"SANS!" Papyrus roared, slamming his boot into the ground angrily. "GET YOUR ASS DOWN HERE RIGHT NOW!"

Sans smirked confidently. This was the moment of truth: he was finally going to teach his brother a lesson for pushing him around. Psyching himself up and putting on his typical anxious expression - as much as he hates to admit, he secretly fears his brother's wrath - he made his way downstairs slowly. "Y-Yeah, Boss?"

"Did you check the spike pits like I told you to?" the younger brother growled.

Shit. Sans was so caught up in his plan on getting back at Papyrus that he totally forgot to check the spike pits! Oh, Papyrus was not going to be happy... "Er, uh... W-Well..."

Papyrus' eye sockets narrowed into tiny black slits of rage, and a small tint of furious rage could be seen flickering in them. "Sans, I _told_ you to check the spike pits! A human could have fallen in, and if they did their SOULs would have burst long ago! You could have cost all the monsters in the Underground their only chance at escaping this wretched place for the next few years!"

"C-C'mon, Boss, I'm trying," Sans tried to reason, feeling his knees begin to tremble despite himself.

" _Trying_ is not _doing_ , Sans! Honestly, what else could you possibly screw up, huh? And stop looking at me like I'm gonna kill you, I'm not gonna do that! Not yet anyway."

Sans felt his bottled up fury boiling over again, and he narrowed his sockets into tiny red slits as his teeth gnashed together. He was finally going to vent out his years of anger towards his brother in a way that can only have a positive outcome on his behalf and a negative on Papyrus'.

"Y'know... _brother_..." Sans growled lowly. "I'm doing my damn best to please ya and do things your way, and I'm hardly gettin' any respect or recognition for it. So frankly, I'm pretty pissed off about it."

"Excuse me?" Papyrus snarled, his sockets widening in rage and surprise. "You dare speak to me that way?"

"Yeah, I dare! You don't even care about the things I do right, only the things I do wrong! Well, I'm sick and tired of it, so I think I'm just going to force some respect outta ya..." Sans grinned evilly as he began wiggling his fingers threateningly, inching his way closer to Papyrus. He was pleased to see him taking a step back in alarm.

"Sans, what are you doing?" Papyrus growled, a trace of nervousness showing in his expression. He was used to getting into arguments and fights with Sans, but he wasn't too used to his brother actually trying to stand up for himself like this. It was new to him, and he didn't know how to properly react.

With a war cry, Sans pounced on Papyrus and knocked him down to the ground, the older brother receiving a cry of alarm from the younger. Sans activated his magic to summon a few crimson-red bones, which he used to pin Papyrus' arms and legs down to the ground. Ignoring his younger brother's demands to be set free, Sans smirked devilishly as he began clawing his fingers up and down his exposed spine. He smirked when he felt Papyrus stiffen underneath him, then grinned devilishly when he burst out laughing.

"A-AAAGH! NYAHAHAHAHA! SANS, YOU LITTLE F-FAHAHAHA!" Papyrus roared with laughter, struggling to yank his arms free, and he managed to shoot a really nasty glare at his older brother. "ST-STAHAHAHAP THIS! STOP THIS NOHOHOW!"

Sans gulped, a pang of alarm shooting through him; he had underestimated his younger brother's strength. He divided his attention between keeping Papyrus' limbs pinned and skittering his fingers across his spine. This wasn't very good on his behalf; it's only a matter of time before he broke free. But at least Sans was finally doing something to stand up for himself, showing Papyrus he wasn't the weak little coward he was always made out to be. Besides, it was pretty satisfying to watch the brother who's been tormenting him for years being reduced to a little giggling fit beneath him, his cheekbones flushed bright red with what he hoped was just humiliation.

"N-No way, bro!" Sans barked back, trying to swallow down his growing anxiety. "You've had this comin' for a long time!" Regaining a bit more of his confidence, he picked up the tickling pace, poking and prodding around the dips and grooves of his brother's vertebrae.

"AHAHAHAHAHAHA! Y-YOU'RE SO DEHEHEHEAD! NYAHAHAHAHA!" the taller skeletal monster howled, bouncing and wiggling his spine around, trying to squirm out of Sans' reach. It was true, he felt very humiliated, but the feeling wasn't nearly as strong as his boiling rage. And... was that... pride? Sans had managed to catch him off guard after all, something not most monsters could do, and the strength in his bone magic was exceptional if he was able to keep him pinned for this long. Perhaps he deserved this after all...

But that didn't mean karma wasn't about to slam into Sans like a tsunami.

Papyrus let out a surprised gasp as Sans suddenly sprung from his spot next to his torso to pin down his kicking legs. He heard his older brother chuckling evilly as he began quickly unbuckling his boots. No! Papyrus couldn't let him get to his feet!

With a burst of adrenaline, Papyrus let out a shout as he yanked as hard as he could on the crimson bones holding his arms down. Sans' concentration on them had slackened in his mirth, so he was able to easily break free, the magical attacks shattering into millions of glowing red pieces before disintegrating into thin air. He sat up quickly and lashed out to grab Sans by the waist, and he let out a startled cry as the bones around his legs evaporated.

"Now you're going to get it!" Papyrus snarled with a wicked grin. In a flash, he used his legs to pin Sans' arms down to the ground and trapped his legs under his arms, fully exposing his torso.

"N-No! Wait, please, I-I'm sorry!" the older brother wailed in fear. Oh, he really screwed up this time! And now he was in his brother's complete mercy - or rather lack thereof. "I'm sorry, I'm sorryyyYYYAAAHAHAHAHAHAHAHA!"

Sans threw his head back with a loud, uncharacteristically high-pitched squeal as Papyrus dug his fingers into his ghost belly over his white undershirt. He was rarely tickled, if ever at all, so he wasn't at all prepared for the surprisingly strong ticklish sensations that resulted from the onslaught. Not to mention his younger brother's pointy, nimble fingers were absolutely devastating and quick to deal the most agonizing session possible.

"How do you like _this_ , you little rat?" Papyrus spat with a malicious cackle. He may have been proud of his brother's rather unique torture tactic and strength, but karmic retribution was still incredibly satisfying. Plus... it was almost... fun to tickle Sans, even if it was in retaliation driven mostly by anger and justice. His loud, boisterous, high-pitched laughter was funny to listen to, and he was surprised to find himself chuckling to himself. He smirked as he scrabbled his fingers up to his brother's ribs.

"N-NO, NOT THERE! NOHOHOHAHAHAHAHAHA!" Sans pleaded, twisting his torso around, no longer having any control over his body. Oh, God, his ribs were so incredibly sensitive, whether it be to pain, heat, cold, and now apparently tickling. His hands slapped weakly against the ground as tears began swelling up on the corners of his tightly-shut sockets. Oh, how he just wanted this to end! He was such an idiot for challenging his brother!

"Damn, Sans," Papyrus chuckled. "You really do have sensitive ribs, don't you?" Deciding to up his ante, he slipped off his crimson gloves and slid his hands under Sans' undershirt to scrape and scratch bone against ticklish bone. He found himself laughing softly as Sans' sockets flew wide open in shock, thrashing and bucking around as he cackled and cachinnated uncontrollably.

"AHAHAHAHAHA! PLEASE STAHAHAHAHAP! I'M SORRY! I'M SO SAHAHAHAHARRY! MERCY, PLEHEHEHEHEASE, PLEEEEEASE!"

At long last, after what felt like an eternity of pure hell, the tickling stopped. It was so abrupt Sans hunched forward as he gasped for breath, coughing as he inhaled too much air at once. Once he was reduced to gentle panting, he thumped his head against the ground, staring up blankly at the ceiling. He was relieved to feel Papyrus let go of him, and that's when he fell completely limp.

A few seconds later, Papyrus peered down at his worn-out brother, wearing a very cocky smirk. "You're not dead, are you?"

"N...No..." Sans huffed. He moaned tiredly as he sat up, massaging his ribs and belly, trying to rub away the tingly ghost of the tickles.

Papyrus chuckled lightly, folding his arms victoriously as he knelt down next to Sans. "So... What do you have to say for yourself?"

"I... I'm sorry, bro- I-I mean Boss..." the older brother quickly stammered, pulling his knees close to his chest as he hugged himself for comfort. "I... I was just..."

"You were standing up for yourself," Papyrus finished.

"Y-Yeah..." Sans hung his head and shut his eyes, bracing himself for any scolding or further punishment he knew was coming next.

Instead, what Papyrus said next made his sockets fly wide open in pure surprise: "Your magic's really improving. Good concentration and strength. You could still fare well with more training. As for the element of surprise, you nailed that down right on the head."

Sans quickly turned to look at his younger brother, sockets wide in disbelief. "You...You're... praisin' me?"

"What, did you want me to keep tickling you instead?"

"N-No, Boss!" Sans wrapped his arms around his ribs tighter. "But... th-thanks... And, uh... thanks for not killin' me, too."

"I wouldn't do that to my brother." Papyrus placed a supportive hand on Sans' shoulder and gave him a small smile.

Sans stared at him for a moment in disbelief before returning the smile.


End file.
